Phase equilibria and microstructure investigation of Mg-Gd-Y-Zn alloy system

In order to develop high strength Mg-Gd-Y-Zn alloys, key experiments coupled with CALPHAD (CALculation of PHAse Diagrams) calculations were carried out in the current work to provide critical understanding of this important alloy system. Three Mg-10Gd-xY-yZn (x = 4 or 5, y = 3 or 5, wt.%) were mapped on Mg-Gd-Y-Zn phase diagrams for phase equilibria and microstructure investigation. Electron microscopy was performed for phase identification and phase fraction determination in as-cast and solution treated conditions. In all three alloys, the major phases were Mg-matrix and long period stacking order (LPSO) 14H phase. With ST at 400 and 500 degrees C, the phase fraction of LPSO 14H increased, particularly the fine lamellar morphology in the Mg matrix. The as-cast and 400 degrees C Mg 10Gd5Y3Zn samples had Mg-5(Gd,Y) present. At 500 degrees C, Mg-5(Gd,Y) is not stable and transforms into LPSO 14H. The Mg 10Gd5Y5Zn alloy included the W-Phase, which showed a reduction in phase fraction with solution treatment. These experimental results were used to validate and improve the thermodynamic database of the Mg-Gd-Y-Zn system. Thermodynamic calculations using the improved database can well describe the available experimental results and make accurate predictions to guide the development of promising high-strength Mg-Gd-Y-Zn alloys. (C) 2021 Chongqing University. Publishing services provided by Elsevier B.V. on behalf of KeAi Communications Co. Ltd.